This invention relates to a method for refining molten steel inexpensively and efficiently and, more specifically, to a method for decarburizing, desulfurizing or dephosphorizing molten steel inexpensively and efficiently and a refining apparatus employed for implementing said method.
Requirements for steel material properties are becoming more and more demanding as steel materials are used in more severe environments. Since steel materials are widely used in the society in general, they are required to be inexpensive, too. For manufacturing steel materials having desired properties, it is necessary to lower impurities such as phosphorus, sulfur, carbon, hydrogen, etc. to the least possible amounts at steel refining processes, and it is also important to refine steel inexpensively. In this situation, it is essential to clarify the physical and chemical fundamentals and principles of steel refining reactions and develop efficient refining methods and apparatuses based thereon.
Conventionally, the technical trend of steel refining has been to divide the refining process into steps so that each of impurities has been removed under a condition tailored to facilitate the removal and to complete the steel refining through several steps. Technologies based on this philosophy have come to be widely practiced. For example, widely employed is a hot metal treatment process wherein the dephosphorizing treatment and the decarburizing treatment, which were formerly carried out using only a converter, have been divided into the dephosphorizing treatment at the step of molten pig iron and the decarburizing treatment in a converter.
At the decarburizing treatment in a converter, carbon is removed through oxidation by injecting oxygen into molten steel (oxidizing refining), but the oxygen is inevitably absorbed in the molten steel.
Oxygen concentration in molten steel becomes high especially when producing low carbon steels having a carbon concentration of 0.1% or less: for example, if blowing is stopped at a carbon concentration of 0.04%, oxygen content in the molten steel will be 0.05% or so. The carbon concentration and the oxygen concentration in molten steel are roughly in inverse proportion to each other and, hence, the lower the end point carbon concentration, the higher the oxygen concentration.
In the meantime, highly formable ultra low carbon steels have come to be used in large quantities especially for exposed panels for automobiles. For producing the ultra low carbon steels, it is necessary to lower the carbon concentration to a level of 30 ppm or less and, for this purpose, decarburizing treatment is carried out by decompression refining at a secondary refining stage after the decarburization in a converter.
At the present time, when the continuous casting method has become general, in order to prevent the occurrence of pin holes and breakouts caused by CO gas generated during casting, it is necessary to remove oxygen absorbed in molten steel by adding a deoxidizing agent, typically Al, to molten steel and trapping the oxygen as oxides. When the deoxidizing agent is entrapped in steel materials, however, it will undesirably cause cracks and defects when they are plated.
Further, the deoxidizing agent remaining in steel materials tends to appear as inclusion-induced defects in the case of low carbon steels often used as materials for stamping applications undergoing intensive working. A process to produce low carbon steels with low oxygen concentration, therefore, needs to be developed.
In this respect, a method called the carbon deoxidation method is widely known, wherein the oxygen in molten steel is removed in the form of CO gas by carbon in the molten steel. In this method a vacuum degassing apparatus equipped with a large evacuator (for example, an RH vacuum degasser) is generally employed for an effective decarburizing action.
Japanese Unexamined Patent Publication No. S53-16314, for example, discloses a method to produce Al-killed molten steel for continuous casting use wherein the end point carbon concentration at a converter is controlled to 0.05% or more and a degassing treatment is applied using a vacuum degasser before deoxidation. By this method, the pressure inside a vacuum tank is controlled within the range of 10 to 300 Torr in accordance with the progress of decarburization. Further, Japanese Unexamined Patent Publication No. H6-116626 discloses a decarburization method, with a reduced occurrence of splash, wherein molten steel in a ladle with carbon concentration reduced in a converter to 0.1 to 1.0% is decarburized by immersing a single cylindrical immersion tube into the molten steel and injecting oxygen mixed with an inert gas under a pressure of 100 Torr or more.
The methods disclosed in the Japanese Unexamined Patent Publication Nos. S53-16314 and H6-116626, however, employ so-called large decompression refining apparatuses. In the method of the Japanese Unexamined Patent Publication No. S53-16314, it is necessary to reduce the pressure to 10 Torr or so, and hence a large vacuum degasser such as a vapor jet vacuum pump is indispensable. In the method of the Japanese Unexamined Patent Publication No. H6-116626 wherein oxygen gas mixed with an inert gas is used for decarburization, on the other hand, there is a problem that expensive argon gas has to be used since, when inexpensive nitrogen gas is used instead, it is absorbed in steel adversely affecting its aging properties.
At the present time, when vacuum degassers are widely used for the purposes of decarburization and dehydrogenation of ultra low carbon steels, the degassers originally designed for degassing at a high vacuum of 1 Torr or less are often used for the production of low carbon steels. However, a high decompression refining apparatus such as an RH vacuum degasser (hereinafter sometimes called xe2x80x9can RH refining apparatusxe2x80x9d) has a vacuum tank very large in height and diameter and, consequently, the volume to be evacuated is huge. For this reason, there are problems of high refining costs due to high unit consumption of refractories and high costs of utilities such as steam for a vapor jet vacuum pump required for evacuation.
Another problem is that the construction of a large decompression refining apparatus intended for the carbon deoxidation of low carbon steels is expensive and uneconomical. Further, a high decompression refining apparatus is used for producing ultra low carbon steels with a carbon concentration of, for example, 30 ppm or less and, in this case, skulls of a high carbon concentration which adhered onto the inner wall of a vacuum tank when molten steel with a carbon concentration of 0.04% or so, which is a far higher carbon concentration than an ultra low carbon steel, is processed, re-melt during the processing of an ultra low carbon steel and become the source of carbon contamination. This leads to another problem of longer decarburizing treatment time or no progress in decarburization. Some RH refining apparatuses are equipped with an LPG burner for melting and removing the skulls as a countermeasure, but such a countermeasure leads to another problem of additional costs for the equipment and the removal operation.
Looking at the desulfurizing treatment of molten steel, it is classified, generally, into hot metal desulfurization applied in the state of molten pig iron and molten steel desulfurization applied in the state of molten steel. As steel materials came to be used in more severe conditions, the required level of steel purity becomes higher. As a consequence, the application of only the hot metal desulfurization can be regarded insufficient and the molten steel desulfurization is an indispensable process step. Thus, the development of a method for efficient desulfurization and an apparatus therefor, especially for producing ultra low sulfur steels having an S concentration of 10 ppm or less, has been required.
As a response, for example, Japanese Unexamined Patent Publication No. S58-37112 proposes a method to immerse an immersion tube (the upleg snorkel of an RH refining apparatus) equipped with a powder injection lance into molten steel in a ladle, and to inject a desulfurizing agent together with a carrier gas toward the immersion tube.
However, although it is possible to lower the S concentration of molten steel to 10 ppm or less by this method, a treatment process employing such a vacuum degasser has a problem of high operation costs for steam, electricity, etc., because a vacuum degasser such as an RH refining apparatus has a huge evacuator for maintaining a high vacuum of 1 Torr or so. There is another problem of high refractory costs because the vacuum degassing tank has to be very tall and large to cope with the violent splashing occurring during the course of the processing.
A ladle refining vessel such as an LF is also capable of reducing the S concentration of molten steel to a level attainable by the RH process, i.e., 10 ppm or less, but this method has problems of high operation costs and a low productivity due to the protracted processing time.
As another solution, a desulfurization method has been proposed wherein an immersion tube equipped with a powder injection lance is immersed into molten steel in a ladle and a desulfurizing agent is injected together with a carrier gas. Although lower in operating cost than the desulfurizing treatment using an RH apparatus, the proposed method accelerates resulfurization by the agitation of slag, which has no desulfurization capability, on the molten steel surface and it is difficult to stably produce ultra low sulfur steels with an S concentration of 10 ppm or less.
Next, looking at the dephosphorizing treatment of molten steel, the degassing and dephosphorizing method proposed in Japanese Unexamined Patent Publication No. S62-205221 can be cited as an example of conventional methods to dephosphorize molten steel. The method is characterized by injecting a dephosphorizing agent in powder from into molten steel having 100 to 800 ppm of free oxygen through a powder injection tuyere provided at a lower part of a vacuum degassing tank. However, since a characteristic of the vacuum degasser employed herein is such that a decarburizing reaction takes place in parallel with the dephosphorizing reaction and the decarburizing reaction proceeds preferentially, there is a shortcoming that the dephosphorizing reaction speed is lowered.
Facing this situation, Japanese Unexamined Patent Publication No. H2-122013 proposed a new degassing and dephosphorizing method, which was characterized in that the degree of vacuum in a degassing tank was controlled during the degassing and dephosphorizing process in accordance with C concentration level of molten steel. Because of a characteristic of an RH vacuum degasser herein employed, however, the control range of the degree of vacuum where the molten steel processing is viable is usually 150 Torr or less, and the decarburizing reaction proceeds still preferentially at this level of degree of vacuum. Although the proposed method is superior to the method proposed in the Japanese Unexamined Patent Publication No. S62-205221 in terms of dephosphorizing reaction, it has a problem that a sufficient dephosphorizing speed is not obtained. Another problem is that, in the case of refining a low carbon steel under the above degree of vacuum, C concentration lowers beyond a target concentration according to a product standard, and a supplementary addition of carbon-containing alloys is required after dephosphorizing treatment, leading to increased alloy costs, longer processing time, etc. There is yet another problem with the method that, since the degree of vacuum is controlled in accordance with the C concentration level in the molten steel, the molten steel surface in the ladle fluctuates largely, making the operation difficult.
Further, the problem of high operation costs for steam, electricity, etc. persists with the methods disclosed in the Japanese Unexamined Patent Publication Nos. S62-205221 and H2-122013, since a huge vacuum degassing tank such as that of the RH vacuum degasser is employed therein. These methods also have the problem of high refractory costs, since they have to use a vacuum degassing tank having a sufficient height to cope with the violent splashing during processing.
An object of the present invention is to solve the above problems of conventional decarburizing treatments and provide a refining method and a refining apparatus capable of producing low carbon steels efficiently and inexpensively, and the gist of the present invention is described in items (1) to (3) below.
(1) A method for refining molten steel by immersing the lower opening end of a cylindrical immersion tube equipped with a lance into the molten steel contained in a ladle, controlling the pressure in the cylindrical immersion tube to a prescribed pressure range to suck up the molten steel, injecting an agitation gas from the bottom of the ladle towards the surface of the sucked-up molten steel, and decarburizing and refining the molten steel under a reduced pressure, characterized in that the method comprising the steps of; controlling the pressure Pt (Torr) in the cylindrical immersion tube so as to satisfy the following formulae (1) and (2), blowing oxygen gas to the surface of the molten steel through the lance, and decarburizing and refining the molten steel under a reduced pressure;
Pt greater than 760xe2x88x921.297xc3x97107/Dc2xe2x80x83xe2x80x83(1)
K=1.71xc3x97Dl0.211xc3x97Dc0.438xc3x97Wmxe2x88x921.124xc3x97Qg0.519xc3x97Ptxe2x88x920.410 greater than 0.046xe2x80x83xe2x80x83(2)
wherein,
K: capacity coefficient concerning the decarburizing reaction (l/min.)
Dl: inner diameter of the ladle (cm)
Dc: circle-reduced diameter of the cylindrical immersion tube (cm)
Wm: mass of molten steel per processing (t)
Qg: quantity of agitation gas injection (Nm3/h.).
(2) A method for refining molten steel according to item (1), characterized by receiving, in a ladle, molten steel having a carbon concentration higher, by 0.03 to 0.06 mass %, than a final target carbon concentration of 0.02 to 0.06 mass % and decarburizing the steel under a reduced pressure.
(3) An apparatus for refining molten steel by providing a cylindrical immersion tube whose lower opening end is immersed into the molten steel above a ladle containing the molten steel in a manner to move vertically, sucking up the molten steel into the cylindrical immersion tube, and decarburizing and refining the molten steel under a reduced pressure, characterized by; a lance for blowing oxygen gas to the surface of the molten steel at the upper portion of the cylindrical immersion tube, a pressure control means for controlling the pressure Pt (Torr) in the cylindrical immersion tube so as to satisfy the following formulae (1) and (2) at the upper portion or a side portion of the cylindrical immersion tube, and an agitation gas injection means provided at the bottom portion of the ladle for injecting the gas from the bottom of the ladle to agitate the molten steel so that said gas passes through the surface of the molten steel in the cylindrical immersion tube;
Pt greater than 760xe2x88x921.297xc3x97107/Dc2xe2x80x83xe2x80x83(1)
K=1.71xc3x97Dl0.211xc3x97Dc0.438xc3x97Wmxe2x88x921.124xc3x97Qg0.519xc3x97Ptxe2x88x920.410 greater than 0.046xe2x80x83xe2x80x83(2)
wherein,
K: capacity coefficient concerning the decarburizing reaction (l/min.)
Dl: inner diameter of the ladle (cm)
Dc: circle-reduced diameter of the cylindrical immersion tube (cm)
Wm: mass of molten steel per processing (t)
Qg: quantity of agitation gas injection (Nm3/h.).
Another object of the present invention is to solve the above problems of conventional desulfurizing treatments and provide a molten steel refining method capable of desulfurizing molten steel efficiently and inexpensively, and the gist of the present invention is described in item (4) below.
(4) A method for refining molten steel by immersing the lower opening end of a cylindrical immersion tube equipped with a lance into the molten steel contained in a ladle, controlling the pressure in the cylindrical immersion tube to a prescribed pressure range to suck up the molten steel, injecting an agitation gas from the bottom of the ladle towards the surface of the sucked-up molten steel, and desulfurizing and refining the molten steel under a reduced pressure, characterized in that the method comprising the steps of; controlling the pressure in the cylindrical immersion tube to the range of 100 to 500 Torr, controlling the injection amount of the agitation gas to the range of 0.6 to 3.0 Nl/min.xc2x7t, blowing a desulfurizing agent in powder form, together with a carrier gas, through the lance to the molten steel surface, and desulfurizing and refining the molten steel under a reduced pressure.
A further object of the present invention is to solve the above problems of conventional dephosphorizing treatments and provide a refining method of low carbon steels capable of dephosphorizing molten steel efficiently and inexpensively, and the gist of the present invention is described in item (5) below.
(5) A method for refining molten steel by immersing the lower opening end of a cylindrical immersion tube equipped with a lance into the molten steel contained in a ladle, controlling the pressure in the cylindrical immersion tube to a prescribed pressure range to suck up the molten steel, injecting an agitation gas from the bottom of the ladle towards the surface of the sucked-up molten steel, and dephosphorizing and refining the molten steel under a reduced pressure, characterized in that the method comprising the steps of; controlling the pressure in the cylindrical immersion tube to the range of 100 to 500 Torr, controlling the injection amount of the agitation gas to the range of 0.6 to 3.0 Nl/min.xc2x7t, controlling free oxygen in the molten steel to 300 ppm or more, blowing a dephosphorizing agent in powder form, together with a carrier gas, through the lance to the molten steel surface, and dephosphorizing and refining the molten steel under a reduced pressure.
A yet further object of the present invention is to provide a refining apparatus for implementing desulfurizing treatment or dephosphorizing treatment according to the present invention and the gist of the present invention is described in item (6) below.
(6) An apparatus for refining molten steel by providing a cylindrical immersion tube whose lower opening end is immersed into the molten steel above a ladle containing the molten steel in a manner to move vertically, sucking up the molten steel into the cylindrical immersion tube, and desulfurizing or dephosphorizing and refining the molten steel under a reduced pressure, characterized by; the cylindrical immersion tube designed so that its height is 3,500 to 7,500 mm and the ratio of its diameter to the ladle diameter is 0.25 to 0.5, a lance for blowing a desulfurizing or dephosphorizing agent in powder form, together with a carrier gas, to the surface of the molten steel at the upper part of the cylindrical immersion tube, a pressure control means for controlling the pressure in the cylindrical immersion tube to the range of 100 to 500 Torr at the upper portion or a side portion of the cylindrical immersion tube, and an agitation gas injection means provided at the bottom portion of the ladle for injecting the gas from the bottom of the ladle to agitate the molten steel so that said gas passes through the surface of the molten steel in the cylindrical immersion tube.